TRILOBITE BIOSTRATIGRAPHY ACROSS THE BASE OF THE WHITEROCKIAN SERIES (ORDOVICIAN) AT ITS TYPE SECTION, WHITEROCK CANYON NARROWS, NEVADA

The Whiterockian Series is the Laurentian expression of the Middle Ordovician Series. The section at the Whiterock Canyon Narrows, Nevada, has been selected as the boundary stratotype with the first appearance (FAD) of the conodont Tripodus combsi defining the base of the series. The nature of the base of the Whiterockian, however, has been controversial with some authors suggesting that the stratotype includes a significant unconformity.

Two parallel sections at the stratotype have been sampled for trilobites and conodonts. The lower beds of the underlying Ninemile Formation have yielded Ibexian faunas (trilobite Zone J, the Pseudocybele nasuta trilobite Zone). Trilobites recovered from the Ninemile include Lachnostoma latucelsum, Ptyocephalus declevitus, and P. nasuta (previously recovered from the Ibex region, Utah).

Whiterockian faunas (Zone L, the Psephosthenaspis trilobite Zone, the Tripodus combsi conodont Zone), previously documented from the overlying Antelope Valley Limestone, are shown to extend locally into the uppermost Ninemile. In comparison to more continuous section in the Ibex region, Utah, the equivalent of the 2 trilobite subzones in the lower Psephosthenaspis Zone and, possibly, the uppermost interval of the underlying P. nasuta Zone are missing.

Contrasts in the distribution of trilobites and conodonts between the 2 parallel sections at the stratotype demonstrate that the unconformity represents the local erosional loss of a minimum of 10m (30 feet) of shale and bedded limestone within the upper Ninemile Formation.